The method for wet gluing is also known as the “blow line” gluing method. In this method, the adhesive is sprayed into the conveyor pipe or blow pipe (blow line), within which the transport of the fibres from the refiner to the dryer takes place. Wet gluing is described in detail on pages 81 to 84 of “Deppe/Ernst, MDF—Mitteldichte Faserplatten” DRW-Verlag, ISBN 3-87181-329-X. The principle consists of spraying the adhesive (glue) into a stream of fibres, which moves at a speed of 150 to 500 m/s. In the device for wet gluing previously disclosed in DE 20 2008 015 419 U1, the speed of the fibres is further increased by a reduction in the cross section of the blow pipe at the injection point. The glue nozzles are arranged perpendicularly to the fibre transport direction or at an acute angle to the blow line. Also previously disclosed are arrangements in which a plurality of gluing nozzles are used, which are arranged in the form of a ring in the gluing zone.
Urea formaldehyde resins (UF) are used primarily as adhesives. As an alternative, these resins can also be reinforced with melamine and/or phenol (MUF, MUPF), and methylene diphenyl diisocyanates (MDI) are also designated as adhesives for wood material boards intended for special applications.
In the case of a blow line with a constant cross section, the adhesive must be dispensed in a relatively short section of the line, as a result of which it is not possible for a homogeneous application of glue to occur in the stream of fibres, which can result in a shading effect. Because of the nature of the process, no additional vaporization energy can be applied through a propellant, which on the one hand results in an unfavourable distribution of the adhesives on the individual fibre and on the other hand leads to an uncontrollable droplet size. This ultimately results in a susceptibility to clogging at the point of discharge through large openings and, as a consequence, the possibility of the admission of fibres against the direction of flow of the adhesive.
If the cross section in the blow line is made smaller, the droplet size will in fact be reduced through the ability to use a further medium in conjunction with the atomization, as described in WO 2009/116877 for example. However, the more compact stream of fibres intensifies the shading effect after the reduction in the cross section, which can be penetrated only with difficulty and inadequately by the adhesive stream. As a result of an increase in the speed, the time spent by the fibres in the gluing zone is shortened, and it may then become necessary to increase the quantity of adhesive in order to achieve adequate gluing of the individual fibres. In this process, the refiner is also required to overcome a greater counter pressure (additional bottleneck in the gluing zone), as a consequence of which the maximum throughput volume of the plant as a whole is reduced. The effectiveness of the propellants is reduced by the higher counter pressure in the gluing zone and the correspondingly low difference in pressure between the adhesive that is forced through the nozzles by the propellant and the pressure in the stream of fibres.
On grounds of cost alone, an effort is made to save glue. In addition, there is a risk of (excess injected) glue that is not able to combine with individual fibres becoming deposited on the walls of the blow line and causing these to become clogged in time.
Previously disclosed in DE 199 30 800 A1, in conjunction with the dry application of glue to fibres, is the reduction of the transport speed of the fibre mixture by an increase in the flow cross section of the tubular dryer and the resulting generation of a turbulent flow, of which the turbulence is increased by the injection of additional air when the glue is injected.
In order to ensure a fine wetting in conjunction with the dry application of glue, previously disclosed in EP 0 728 562 A2 is the arrangement of the nozzles in a region of the conveyor pipe, which region exhibits a diffuser-like enlargement of its cross section.